Five Tempe sewage plant workers became ill when sewer gas surged to dangerous levels — and while the city had been alerted this kind of event was likely, it did little to prevent it.

City employees warned of a “very high potential” for sewer gas to reach unsafe levels more than a year before a September incident in a building where raw sewage flows for treatment.

The employees and a consultant outlined ways the city should reduce sewer gas, vent it from the building and better warn workers when levels become dangerous, according to documents released to the Tribune through a public records request.

The records show sewer gas levels had shot up to 30 times the allowable level in the past and that the city should expect future problems.

City officials acknowledged that they didn’t pursue most of the recommendations that could have prevented the workers from becoming ill.

“Obviously, that’s something that slipped through the cracks,” said Don Hawkes, Tempe’s water utilities manager.

Now, the city is scrambling to fix problems that led to the gas exposure on Sept. 6 and 7. It’s unclear yet if any employees will face discipline for not addressing problems the city had been warned of.

“That’s still under review,” said Jeff Kulaga, interim city manager. “Goal No. 1 here is to improve the safety. Let’s make it safe first and then worry about other things second.”

The city employees who became ill haven’t been doing maintenance in the building recently because of the situation. Until some improvements are made, the city is using a private contractor to perform duties in the building. Those workers are wearing breathing equipment to ensure they don’t inhale the air in the building — even when sewer gas levels aren’t high.

The employees became ill after trying to block sewage that flows through a 12-foot-deep channel so they could inspect a damaged screen that filters the flow.

The employees struggled to place barriers in place, partly because the plant had been renovated recently and they’d never done this before.

As they were moving the barriers, a portable alarm sounded, indicating that hydrogen sulfide levels had reached 10 parts per million, a level at which the city requires employees to leave the building.

The alarm went on and off repeatedly as gas levels varied, and employees weren’t always able to leave immediately. The highest level reached 26 parts per million.

All of the employees were ill after they went home on the second day, which was a Friday. Nobody linked the illness to what happened at the plant until the next week, when they realized they’d all had diarrhea, headaches, sore eyes, fatigue and breathing problems.

These are typical symptoms of poisoning from hydrogen sulfide, which gives sewer gas its odor. The substance can cause respiratory failure or even death, but at many times the employees’ exposure level.

The gas levels had reached 300 parts per million in July 2006, a level that can cause respiratory failure, a coma or even death. Nobody was in the building at the time.

A city investigation found numerous problems, including:

• City employees identified some issues in a health and safety evaluation in 2006, when they recommended a permanent monitor so employees could determine gas levels before going into the building. Portable detectors can take more than a minute to get an accurate reading, so employees could be exposed to dangerous levels before the detector would alert them. The city did not install the permanent detector.

• The city purged old readings of gas levels, which could violate various regulations.

• The city failed to develop procedures or evaluate work hazards, despite an employee call to do so.

• City officials don’t know what level of air filtration is needed at the plant, even after the incident. The plant may be required to exchange the air 12 times an hour, but two studies found it’s less than that.

• Tension between workers and management triggered communication problems.

• A consultant advised Tempe in 2006 to monitor gas levels in the sewers and to add chemicals that will reduce gas levels before the sewage reaches the plant. The city has done this periodically in the past.

Tempe had explored some of the recommendations, Kulaga said. The city didn’t pursue all of them at once because one or two of the fixes may have prevented the city from spending money on other things, Kulaga said.

“You don’t attack the whole problem at once,” Kulaga said. “You’ve got to look at things one at a time to track results.”

Hawkes said employees aren’t in the building all the time, so officials are looking to schedule work inside of it when gas levels are typically lower. Also, the city can divert the flow to another plant during some work, which would cut the source of the gas.

Hawkes said the chemical treatments can help, but they also disrupt the process of sewage treatment. The air filtration could cost $250,000 and still not lower levels in extreme cases, Hawkes said.

“Ninety-nine percent of the time, the atmosphere in there is OK,” Hawkes said.

The city is doing several things now. It bought a $15,000 permanent monitor that could be installed this week. A consultant is evaluating air filtration. Workers are drafting procedures that hadn’t been set before. Hawkes said he’s talked with the employees and is working to improve communication.

“I assured them and made my commitment to them that we will work through this to make sure they are safe,” he said.